1. Field of the Invention
This invention relates to a plasma processing apparatus and a processing method making use of the same. More particularly, it relates to a plasma processing apparatus or system by which negative ions can be generated in a large quantity and also the negative ions can be made incident on a processing article or target to etch or clean the processing target to remove unwanted matter therefrom, and a processing method making use of the same. This plasma processing apparatus is preferably used in processes for producing semiconductors such as LSIs, optical devices such as optical disks and waveguides, and magnetic devices such as magnetic disks.
2. Related Background Art
In conventional plasma processing, positive ions have chiefly been utilized, as disclosed in, e.g., Integrated Circuit Processing Technique Series, Semiconductor Dry Etching Techniques, p.41 (compiled by Takashi Tokuyama, Sangyo Tosho K.K.). FIG. 5 is a diagrammatic cross-sectional illustration of an example of a parallel plate plasma etching apparatus conventionally used. In FIG. 5, reference numeral 501 denotes a high-frequency power source; 541, a high-frequency electrode; 509, a semiconductor substrate (processing article or target), 542, an ion source; 543, a plasma; 544, a vacuum vessel; 545, a grounded electrode; and 504, a processing gas inlet. In this apparatus, the electrode 541 to which a high frequency power is to be applied to form the plasma is provided inside the vacuum vessel 544. Also, the processing target 509 to be processed is placed on the electrode 541 to which a high-frequency power is to be applied. Upon application of a high-frequency power to the electrode 541, the plasma 543 is formed between the grounded electrode 545 and the high-frequency electrode 541 which are provided in parallel. Here, a region where electrons are absent, called an ion sheath, is formed between the plasma 543, the high-frequency electrode 541 and the vacuum vessel 544 because of a difference in mobility between ions and electrons in the plasma 543, so that with respect to the electrode the plasma has positive potential on the average. The electrode 541 to which a high-frequency power is applied has a great potential difference with respect to the plasma, compared with the grounded electrode 545, and may have even a difference of hundreds of volts at maximum. The positive ions in the plasma 543 are accelerated by the potential of such a sheath, and are incident on the processing target 509 while having a certain energy. Conventional apparatuses have utilized energy particles composed of such positive ions to carry out etching and cleaning of substrate surfaces.
However, in the processing with positive ions which has conventionally been used, positive electric charges accumulate on the processing target surface during processing. This is ascribable to the lateral-direction velocity difference that is due to thermal motion of ions and electrons, and is a phenomenon that the electrons, which are light, have so high a lateral-direction velocity that they do not reach the bottoms of deep holes, but the ions, which have a large mass, have so low a lateral-direction velocity that they reach the bottoms of deep holes, thus the positive electric charges accumulate on the bottoms of deep holes. Also, such a charging phenomenon is further amplified because of the release of secondary electrons that is caused by energy bombardment of ions. This charging has caused a difficulty that a great electric field which is greater than breakdown voltage is applied to the gate oxide film of a field-effect transistor to cause a breakdown, or a problem that the course of positive ions incident on the processing target is bent by Coulomb force because of the charging of a resist mask to cause etching malformation.
FIG. 6 illustrates an unfinished semiconductor device having a via hole 230. Reference numeral 221 denotes a silicon substrate; 222, a device separating insulating film; 223, a gate oxide film; 224, a gate electrode; 225 and 229, interlayer insulating films; and 227 and 228, barrier metal layers.
When the unfinished semiconductor device having a cross-sectional structure as shown in FIG. 6 is subjected to cleaning with positive ions, native oxide films or crystal defects brought in by ion bombardment at the time of etching remain at the bottoms of via holes for connecting wirings formed on the silicon substrate surface. Hence, if second-layer metal wiring 232 is formed in the state the via holes are left as they are, the native oxide films or crystal defects make the via holes have a high resistance to bring about circuit retardation or wiring faulty conduction, as known in the art. Accordingly, these residual matter must be removed by cleaning or the like. However, since the device having been processed by cleaning is taken out in the atmosphere, native oxide films again grow on the cleaned surface, and hence the cleaning and the formation of the second-layer metal wiring 232 may preferably be carried out while the device is kept in vacuum. As cleaning methods meeting such a demand, methods making use of plasma have widely and commonly been used. What comes into question here is the phenomenon of charge-up caused by plasma. When this cleaning is carried out by conventional positive ion processing, the positive electric charges introduced by plasma flow to the gate electrode 224 through first-layer metal wiring 226, and finally a voltage is applied to the gate oxide film 223 present between the silicon substrate 221 and the gate electrode 224. Once this voltage reaches a breakdown voltage, the gate oxide film results in electrostatic breakdown. Also, a very weak tunnel electric current may flow through the gate oxide film 223 even at a voltage below breakdown voltage to cause a great deterioration of its lifetime. There have been such problems.
As discussed above, only positive ions have ever been utilized in semiconductor fabrication processes and negative ions have little been utilized. Recently, however, for the purpose of solving the problems arising from positive ions, negative ions present in processing plasma have attracted notice. The following methods are proposed as plasma processing that utilize negative ions.
(1) Method making use of time afterglow of plasma:
An apparatus as shown in FIGS. 7A and 7B, disclosed in Japanese Patent Application Laid-Open No. 8-181125, can be given as an example of an apparatus utilizing this method. In FIG. 7A, reference numeral 601 denotes a microwave power source; 602, a magnetic-filed coil; 603, a waveguide; 610, a processing target; 612, a processing target supporting stand; 614, a plasma; 651, a vacuum vessel; and 652, a high-frequency power source. In this method, the plasma 614, which is formed by pulse-modulating microwaves generated from the microwave power source, is made on/off, and, in the period of plasma-off, the plasma temperature is lowered to form negative ions. Also, a high-frequency bias is applied from the high-frequency power source 652 to the supporting stand 612 of the processing target 610 in synchronization with the pulse modulation of the plasma 614 to draw positive/negative ions alternately into the processing target 610 as shown in FIG. 7B, thus the processing target 610 is processed.
(2) Method making use of spatial afterglow to guide plasma spatially downstream:
Not shown in the drawing, this method is a method in which the processing target is placed downstream by tens of centimeters from the region of plasma formation so that negative ions formed while being diffused downstream and cooled gradually are utilized.
The above two methods, however, have had the following problems.
i) In the method making use of pulse-modified plasma, positive ions are formed in the remaining half period where plasma stands "on", and hence a high efficiency is promised for etching apparatuses of positive/negative-ion alternating irradiation. However, in the case of etching carried out by predominantly using negative ions, it is difficult to attain a high efficiency because the negative ions are formed only in the half of the processing time.
ii) In the method where plasma is guided spatially downstream to lower plasma temperature to form negative ions, the recombination of plasma at vacuum vessel walls causes an abrupt decrease in plasma density itself, and hence the plasma formed in a high density can not efficiently be converted into negative ions.
Thus, in these conventional methods, there has been room for improvement in respect of large-quantity formation of negative ions and effective processing.